The aim of this study is to examine in detail both the intrinsic synaptic organization of the visual cortex as well as its extrinsic connections with other cortical and subcortical areas through the use of modern neuroanatomical techniques. During the past two years we have concentrated on the long afferent and efferent connections of the visual cortex by injecting minute amounts of horseradish peroxidase or tritiated leucine or both into either the striate or prestriate cortex of the squirrel monkey. The results indicated that in addition to the reciprocal connections between the dorsal lateral geniculate nucleus and the striate cortex, there are precise reciprocal connections between the pulvinar (both lateral (PuL) and inferior (PuI) subdivisions) and the prestriate cortex. In addition, the striate cortex sends a minor projection to both PuL and PuI as well as a prominent projection to the superficial portion of the stratum griseum superficiale of the superior colliculus. Our recent studies with intrageniculate injection of 3H-leucine indicated that at least portions of the LGN project to prestriate cortex as well. Thus, the two "parallel" systems are closely interrelated at both subcortical and cortical levels. As for cortico-cortical interconnections, the striate cortex sends a topographical projection to adjacent prestriate cortex and to MT (middle temporal area), while the prestriate cortex projects to adjacent peristriate cortex (the "third tier" of Allman and Kaas), MT, as well as reciprocally back onto the striate cortex. Such precise and complex interconnections presumably assure optimal information processing among cells of identical or related functional properties.